1. Field of the Invention
The present invention relates to method used to fabricate semiconductor devices, and more specifically to a method used to eliminate the formation of voids, in aluminum based interconnect structures, created during photoresist removal procedures.
2. Description of the Prior Art
Aluminum, or aluminum based metallizations, have been extensively used by the semiconductor industry, for wiring or interconnect structures. The ability to easily deposit aluminum, using either sputtered or evaporation procedures, as well as the ability to easily pattern aluminum layers, using high density plasma etching, (HDPE), procedures, have resulted in the recurrent use of this metal in advanced semiconductor type products, such as metal oxide semiconductor field effect transistors, (MOSFET). However the inability of aluminum to withstand high current densities required the addition of small levels of copper, usually between about 1 to 5%, to be included in the aluminum layers. The Al--Cu alloy resulted in dramatic improvement in electromigration resistance, when compared to pure aluminum counterparts. In addition to copper in aluminum layers, small levels of silicon, between about 0.5 to 1.0% are also added to the alloy, to reduce the risk of aluminum penetration, into the semiconductor substrate, that can occur during elevated temperature cycles.
The addition of copper, to aluminum layers, although resulting in electromigration resistance improvements, increased the risk of specific type of corrosion mechanisms, of the Al--Cu alloy, compared to the risk of corrosion encountered with pure aluminum layers. For example during the deposition of Al--Cu alloy, a theta phase of Al.sub.2 Cu precipitates are formed, highly rich in copper, and surrounded by regions of aluminum, that have almost been completely depleted of copper. This inhomogeneity, in the aluminum based layer, can result in a galvanic cell in which the Al.sub.2 Cu precipitates behave as the cathode, while the surrounding aluminum rich regions behave as the anode. Therefore the presence of an electrolyte can then result in galvanic corrosion, or a redox reaction, in which Al will be oxidized, while the Cu is reduced. The Al.sup.3+ ions produced during this reaction, can be leached away during subsequent water rinses. Since this galvanic reaction is localized near the Al.sub.2 Cu precipitates, the result of this galvanic reaction is the formation of voids in the aluminum layer. The aluminum based layer, containing voids, is now less resistant to deleterious electromigration phenomena, as well as exhibiting a decrease in conductivity.
The electrolyte needed to initiate the galvanic reaction, can be water, used for post-clean rinsing procedures. For example patterning of an aluminum based layer, comprised of Al.sub.2 Cu precipitates, as well as aluminum rich regions, is usually performed using HDPE procedures, and using a photoresist shape as a mask. After completion of the patterning procedure the photoresist shape is removed via a strip process; first an H.sub.2 O vapor passivation strip, and than an O.sub.2 plasma ashing step. These procedures are followed by a water rinse, performed at a temperature between about 20 to 25.degree. C. The exposed sides of the aluminum based structure, with the exposed Al.sub.2 Cu precipitates, and the exposed aluminum rich region, in a water environment, can result in the initiation of the deleterious galvanic reaction, creating the unwanted voids in the aluminum based interconnect structure.
Solutions to the galvanic corrosion reaction, during photoresist removal procedures, have been offered by Konno, et al, in U.S. Pat. No. 5,397,432, as well as by Chen, et al, in U.S. Pat. No. 5,545,289. These inventions offer passivation of the exposed sides of the aluminum based interconnect structure, during photoresist post-clean procedures, via the creation of a passivating oxide layer, formed prior to exposure to water post-cleans. cleans. However the creation of these passivating layers can be thin, and porous, with the quality of the native oxide layer, formed during the H.sub.2 O vapor passivation step, being marginal. Therefore these passiavting layers may not totally protect against galvanic corrosion during the water post-clean steps. This invention will describe a photoresist removal procedure in which a pre-heat procedure is performed in a high pressure oxygen ambient, prior to the H.sub.2 O vapor passivation step, and prior to the subsequent ashing step. This procedure is performed at a temperature, and for a time, needed to create a passivating layer on the sides of the exposed aluminum based interconnect structure. In addition this invention will offer a post-clean DI water rinse, at a temperature between about 5 to 10.degree. C. Since reaction rates approximately doubled with every 10.degree. C. increase, the use of this lower temperature will reduce the galvanic corrosion rate by about 2 to 4 times, if porosity does exist in the passivating sidewall layer.